The map moves and this does not, which the map alone cannot express. The
scroll registers move ALL of it, so a score printed into the map slides
away, and one printed into whichever rows the view happens to be showing
jumps a pixel at a time as the fine offset changes.
Port 0x3D is how many rows tall and 0x3E is which row it starts at. Nought
tall is no window, so a cleared screen has none and every program written
before this means what it meant. A start row is a register because a
status bar along the bottom is as common as one along the top.
IT HAS ITS OWN MEMORY, and that is the argument for it. The cheaper design
draws the top rows of the MAP without the scroll applied - no new memory,
one register - and makes those rows part of the playfield's ring, so a
game that scrolls vertically has to route its world around its own
scoreboard for ever. The point of a status bar is that it is not somewhere
in the level. Lunar Porter does not scroll vertically today and will the
moment an orbit is a thing you can reach.
0xC000 in the screen bank, which the map does not reach: it ends at
0xBFFF. Same cells, same tiles, same pages, same schemes. Being in the
screen bank makes it per screen, so flipping the buffer flips the status
bar with it - what a double buffered game wants, and surprising the other
way round.
Drawn over everything, sprites included. A sprite that could cover the
fuel gauge would be a bug in every game that had both.
Tile modes only. In bitmap mode the picture is using that memory, so a
bitmap program pins things to the screen with sprites, which are in screen
coordinates for the same reason.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
AnachronautandClaude Opus 5
17c8da111f